1. Field of the Invention
This invention relates to the field of extractive metallurgy and is directed particularly to the recovery of alumina from clays. Specifically, the invention deals with the recovery of alumina from calcined clay by a pressure hydrolysis technique.
2. Description of the Prior Art
Prior art processes for recovering alumina from clays, such as kaolin, have conventionally involved the initial leaching of the calcined clay with a mineral acid, generally nitric acid, in order to dissolve the aluminum as aluminum nitrate. The iron contained within the clay is also leached and must be separated from the leach liquor prior to the recovery of the aluminum.
The resulting Al(NO.sub.3).sub.3.sup.. 9H.sub.2 O crystals are recovered from the purified leach liquor by removing enough water to cause crystallization when the solution is cooled. The Al(NO.sub.3).sub.3.sup.. 9H.sub.2 O can be decomposed to Al.sub.2 O.sub.3 either by direct roasting or a two-step process wherein a basic aluminum nitrate is formed at a low temperature, around 150.degree. C, and the basis nitrate is then roasted to Al.sub.2 O.sub.3. The disadvantages of crystallizing Al(NO.sub.3).sub.3.sup.. 9H.sub.2 O and roasting it is that energy consumption is very large and a large heat input is required for vaporization of the excess water and for roasting the Al(NO.sub.3).sub.3.sup.. 9H.sub.2 O.
Another process for making alumina monohydrate, ALOOH, as an intermediary product for the ultimate production of Al.sub.2 O.sub.3 is disclosed by U.S. Patent Application Ser. No. 702,262, filed on July 2, 1976 and now U.S. Pat. No. 4,044,115. According to this process, the alumina monohydrate is produced directly from the purified aluminum nitrate leach liquor without evaporation of water, thereby substantially reducing energy consumption. In addition, the ALOOH may be roasted to Al.sub.2 O.sub.3 with considerably less energy consumption than that required for roasting of Al(NO.sub.3).sub.3.sup.. 9H.sub.2 O. This consists of treatment of the leach liquor, or other aluminum nitrate solution, at a temperature of about 250.degree. to 350.degree. C and a pressure of about 500 to 2500 psig for a time sufficient to convert a major proportion of the aluminum nitrate to alumina monohydrate according to the reaction 2Al(NO.sub.3).sub.3 (aq) + 4H.sub.2 O(lq) .fwdarw. 2AlOOH(c) + 6 HNO.sub.3 (aq).
Leach liquors suitable for treatment according to this process will typically consist essentially of aqueous solutions of aluminum nitrate nonahydrate containing about 0.5 to 7.2 weight percent of aluminum. The process is preferably carried out in an autoclave by means of which the required pressure is produced autogenously when the temperature is adjusted to the required value. Optimum reaction time will vary with temperature and pressure, as well as concentration of the solution, but generally a period between about 1 and 6 hours will be sufficient. The reaction mixture is then cooled to room temperature and the granular precipitate of AlOOH separated by conventional means such as filtration. The AlOOH may be readily converted to Al.sub.2 O.sub.3 by roasting at a temperature of about 900.degree. to 1300.degree. C. The recovered HNO.sub.3 can be recycled to leach additional clay.
Other examples of prior art processes for the production of alumina from clays are exemplified by U.S. Pat. No. 3,211,521 and U.S. Patent Application Ser. No. 639,024, now